In general, passenger cars which employ an automatic transmission of the hydraulic type have a torque converter on board. On the other hand, special motor vehicles used for specific purposes, such as a dump trucks or fire engines, employ a power take off device with the transmission system for supplying auxiliary power for functions other than moving the vehicle.
A conventional power take off device for an automatic transmission has been proposed in Japanese Patent Publication No. 6(1994)-74848. This publication discloses a lock-up control device that locks up the transmission when a power take off device is turned on while a shift lever position is in the P (parking) position or the N (neutral) position. Accordingly, it is possible to effectively use the engine power.
However, in this device, in situations where the solenoid valve or the lock-up control valve does not operate because of a valve sticking while the transmission is locked up, the lock-up condition is kept on even if the power take off switch is turned off. In the above situation, when the shift lever is moved from the P (parking) position to the D (drive) position for running the vehicle, the transmission changes the gears from the neutral stage to the first stage. The engine may stop because of the large shock to the engine that is caused by the lock-up condition of the transmission.